Chapter 141:8Laws of the Megillah

8. It is forbidden to read the Megillah at night before the appearance of
three stars ("Tzeis HaKochavim"), even though one is suffering great
discomfort due to the fast [of Esther]. However, one may consume a little
(1) before the Megillah reading, such as coffee and similar beverages, in
order to strengthen oneself from the exhaustion of the fast.

FOOTNOTES:

(1) One may eat the volume of a "k'beitzah" (lit: "like an egg") of bread
and cake, or liquid in the same quantity (Mishna Berura 692:14). Opinions
as to the size of a "k'beitzah" range from 86.23 ml to 100 ml. In more
concrete terms, a "k'beitzah" is two thirds of a slice of white bread or
three quarters of a square matzah (Halachos of K'zayis by Rabbi Y.
Bodner).